Light Profile Microscopy Apparatus and Method

ABSTRACT

An apparatus and method allowing an optimized illumination in a light profile microscope by excitation of a sample with an elliptically collimated beam. The beam, which is typically supplied by a laser is collimated with unequal beam waist radii (and Rayleigh ranges) along major and minor axes orthogonal to a propagation direction, and approximates a plane sheet of illumination. The plane sheet of illumination is aligned with a thinnest width dimension thereof along the optic axis of the microscope objective, and with a center thereof at the object plane of the objective. Excitation light in a test sample is thereby confined to within a narrow thickness of the object plane of the objective lens, which minimizes out-of focus light in the image. The major axis width of the plane illumination sheet is typically a factor of ten or more greater than the minimum width, allowing a large area of the test sample to be illuminated and imaged. This excitation arrangement optically emulates the operation of micro-toming a thin cross section of a material for analysis, and provides optimum resolution and field in a light profile microscope.

FIELD OF THE INVENTION

The present invention relates to light profile microscopy (LPM). Morespecifically, the present invention is concerned with an apparatus and amethod for illuminating a test material in a light profile microscope,so as to provide an optimized image resolution under broad fieldconditions.

BACKGROUND OF THE INVENTION

Light profile microscopy is an imaging method for obtaining directimages of cross sections of thin films using a light profile microscope(LPM) (see for example U.S. Pat. No. 6,614,532). The cross sectionalimages thus obtained may be used to identify the number, dimensions,composition and/or material morphology of individual layers making upthe structure of a thin film test material for example. Light profilemicroscopy is to date a unique method for direct micrometer scaleimaging of depth structures in thin layers, which may be conductedrapidly and with minimal modification and/or preparation of the testmaterial. Light profile microscopy furthermore allows an imageresolution that is close to or equal to the Rayleigh diffraction limitof an optical microscope as defined herein below (see relations 5 and 6below).

In a standard set up for light profile microscopy, a material undertest, referred to hereinafter as the test material, is usually, althoughnot necessarily, a planar structure with major lateral dimensions and adepth dimension, along axes designated as ‘y’, ‘z’ and ‘x’ respectively.A beam of excitation radiation, referred to hereinafter as the sourcebeam, is directed through the test material along the depth dimension‘x’. This beam is displaced along the ‘z’ axis behind a cross sectionalsurface of the test material, which may be an edge prepared by cleavageto expose the cross sectional structure of the thin film. This surfaceis referred to as the image transfer (IT) surface because radiationtransferred through it is used to form an image of the excitation beampropagating behind it, inside the test material.

The IT surface is usually polished to prevent any optical defectspresent thereon from affecting the image. Image radiation is emittedfrom the source beam in the test material by light scattering, which maybe elastic Rayleigh scatter, or inelastic Raman scatter, by luminescence(fluorescence and/or phosphorescence), or by other emission mechanisms,such as blackbody emission, for example, or by chemi-luminescence asexcited thermally by the source beam. The image radiation is typically,although not exclusively, incoherent with the radiation in the sourcebeam.

An optical imaging system (OIS) is used to form the light profilemicroscopy (LPM) image from the image radiation. The OIS typicallycomprises a combination of lenses and/or mirrors forming an image at animage plane. The OIS is aligned at ninety degrees to the depth axis ‘x’along a direction normal to the IT surface, which typically correspondsto the ‘z’ direction, hereinafter referred to as the optic axis.

The LPM image is recorded with a spatial resolution that is close to thediffraction limit of the OIS, as set forth hereinbelow. The OIS isassumed to have its image resolution close to the diffraction limit. Itis to be noted that the imaging properties of the OIS differ from thoseof macroscopic scale imaging systems. The latter systems have imageresolution that is far from the diffraction limit and is limitedseverely either by optical aberrations or by the dimensions of the imagepixels of a camera or image recording instrument.

The LPM image has a number of features characteristic of images recordedusing an OIS in such an LPM layout. First, the limited object depth offocus of the OIS allows forming an image of the source beam, with thesource beam aligned at a sufficient distance behind the IT surface,inside the test material, so that any scratches and defects in the ITsurface are held significantly out of focus in the LPM image. It is tobe noted that in the event that this condition is not strictly met, theLPM measurement is not invalid. Second, the orthogonal LPM geometrymaintained between the depth axis ‘x’ and the optic axis ‘z’ yields avery high contrast in the LPM image for interfaces and boundaries in thecross section of the thin film test material. This image contrast ismuch greater than that available from images obtained using othermicroscopy methods known in the art. This orthogonal LPM geometry allowsmaking direct imaging of depth structures in a material. Third, theimage features of structures that scatter light in the direction of theOIS are emphasized in LPM images, in contrast to features that do not soscatter, which allows LPM images to record structures that may appearinvisible in other prior art microscopy methods. This high scattercontrast may also have the effect of rendering insignificant scratchesand defects in the IT surface and their contribution to LPM images thatare recorded when the source beam is close to the IT surface.

A number of applications of light profile microscopy as a method ofindustrial thin film imaging are described in U.S. Pat. No. 6,614,532 bythe present inventor for example, and in recent publications in theliterature (see for example J. F. Power and S. W. Fu, Longitudinal LightProfile Microscopy (LLPM): A New Method for Seeing Below the Surfaces ofThin Film Materials Applied Spectroscopy 53(12), 1507-1519 (1999); S. W.Fu and J. F. Power, Broadband Light Profile Microscopy (BB-LPM): A Rapidand Direct Method for Thin Film Depth Imaging, Applied Spectroscopy 58,96-104 (2004); J. F. Power and S. W. Fu, Dual Beam Light ProfileMicroscopy (LPM): A New Technique for Optical Absorption DepthProfilometry, Appl. Spectros. 58(2), 166-178, (2004)).

However, there is still a need in the art for a method and apparatus oflight profile microscopy providing an optimized image resolution underbroad field conditions.

SUMMARY OF THE INVENTION

More specifically, there is provided an apparatus for illuminating atest material in a light profile microscope, comprising a source ofradiation providing a source beam propagating along a beam axis ‘x’; ananamorphic optical means providing, from the source beam emitted by thesource of radiation, a source beam elliptically collimated over an ‘x’axial collimation region having a distance comprised in a range betweenmicrometers and meters, and having a major elliptic axis oriented alonga first transverse axis ‘y’, and a minor elliptic axis oriented along asecond transverse axis ‘z’; a test material positioned to intersect theelliptically collimated source beam within the ‘x’ axial collimationregion to form an irradiated volume, the test material comprising animage transfer (IT) surface oriented substantially parallel to the ‘x’axis and substantially orthogonal to the ‘z’ axis, the IT surfacetransmitting radiation emitted from the irradiated volume in the testmaterial; an optical imaging system (OIS) forming an image, at an imageplane, of the illuminated volume in the test material from the radiationtransmitted by the IT surface; the OIS defining an object planeconjugate to the image plane and aligned to contain the major ellipticaxis ‘y’ of the collimated source beam intersecting the test material inthe illuminated volume, an object depth of focus of the OIS beingmaintained at a value of approximately at least ⅕ of a radius of theelliptically collimated source beam along the ‘z’ axis in the axialcollimation region of the elliptically collimated source beam; and imagereceiving means receiving the image formed by the OIS in the image planethereof; wherein the apparatus yields a high image resolution and wideimage field.

There is further provided a method for illuminating a test material in alight profile microscope comprising a source of radiation, an anamorphiccollimator, an optical imaging system and an image recording means, themethod comprising the steps of propagating a source beam emitted by thesource of radiation along a beam propagation axis ‘x’; ellipticallycollimating the source beam along orthogonal axes ‘y’ and ‘z’ transverseto the beam propagation axis ‘x’ to yield a elliptically collimatedsource beam over an ‘x’ axial collimation region having a distancecomprised in a range between microns and meters, and having a majorelliptic axis oriented along the transverse axis ‘y’ and a minorelliptic oriented along the transverse axis ‘z’; intersecting theelliptically collimated source beam within the ‘x’ axial collimationregion with a test material to form an irradiated volume in the testmaterial by said intersecting, and aligning an image transfer (IT)surface of the test material substantially parallel to the ‘x’ axis ofthe elliptically collimated source beam and substantially orthogonal tothe ‘z’ axis of the elliptically collimated source beam; collecting animage radiation emitted from the irradiated volume in the test materialand transmitted through the IT surface by an optical imaging system(OIS); forming an image at a fixed image plane with the OIS by aligningan object plane thereof conjugate to the image plane thereof at acentral axis of the irradiated volume in the test material; maintainingan object depth of focus of the OIS at a value that is approximately atleast ⅕ of a radius along the ‘z’ axis of the elliptically collimatedsource beam in the ‘x’ axial collimation region; and recording the imageformed by the OIS in the image plane.

Other objects, advantages and features of the present invention willbecome more apparent upon reading of the following non-restrictivedescription of embodiments thereof, given by way of example only withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the appended drawings:

FIG. 1 is a schematic set-up for light profile microscopy (LPM)according to an embodiment of an apparatus according to the presentinvention;

FIG. 2 is a schematic set-up for light profile microscopy (LPM)according to another embodiment of an apparatus according to the presentinvention;

FIG. 3 is a schematic set-up for light profile microscopy (LPM)according to a further embodiment of an apparatus according to thepresent invention; and

FIG. 4 is a schematic set-up for light profile microscopy (LPM)according to a still further embodiment of an apparatus according to thepresent invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Generally stated, there is provided a method and an apparatus forilluminating a sample in a light profile microscope (LPM) to obtain anoptimized spatial resolution in a recorded image, under broad fieldconditions.

As will be explained further hereinbelow, image properties depend on ageometry of a source beam in the sample, including the size, shape, andalignment distance of the source beam behind an IT surface of thesample. This source beam geometry is in turn dictated by characteristicsof the optical imaging system (OIS) used to form the LPM image.

As described hereinabove, the OIS used by a LPM comprises a combinationof lenses and/or mirrors so arranged as to form an image of the samplein the LPM.

The OIS defines an object plane, and an image plane in which it forms animage of a system object consisting of a field of refracted or emittedlight located in the OIS object plane. Each of the object plane and theimage plane is oriented along transverse directions ‘x’ and ‘y’ andintersects an optic axis ‘z’.

The OIS further defines a focal length, referred to as ‘f, which is afixed distance set by optical design, that relates the distance andlocation of the image relative to the system object according to aconjugate relationship as follows (see Warren J. Smith, Modern OpticalEngineering, Second Edition, 1990, McGraw-Hill, New York):

$\begin{matrix}{\frac{1}{f} = {\frac{1}{z_{0}} + \frac{1}{z_{i}}}} & (1)\end{matrix}$

where z₀ is an unsigned distance of the object plane from a firstprincipal plane of the OIS, referred to as the object distance; andz_(i) is an unsigned distance of the image plane from a second principalplane of the OIS, referred to as the image distance.

The image plane of the OIS is located in a fixed position. The image ofa system object is formed in-focus at the image plane, in conformity torelation (1). An in-focus object is a system object, consisting of afield of emitted or refracted light of which the OIS forms an in-focusimage at the image plane.

The system object of the OIS defines an object field, which is the setof all (x, y) points in the object plane from which light is admitted tothe image plane unobstructed by the field stop of the OIS. The image ofthe OIS comprises an image field that is defined as the set of all (x,y)points in the image plane to which light is admitted from the objectfield unobstructed by the field stop of the OIS, and where each point ofthe image field and each point of the object field are related asfollows:

x_(i)=Mx₀   (2a)

y_(i)=My₀   (2b)

where x_(i) is the ‘x’ coordinate in image plane; y_(i) is the ‘y’coordinate in image plane; x₀ is the ‘x’ coordinate in object plane; y₀is the ‘y’ coordinate in object plane; and M is the lateralmagnification (a constant but signed quantity).

The OIS is a highly corrected optical system, wherein aberrations arecorrected by design to a sufficiently low level that the imageresolution is limited only by diffraction, and relations (1) and (2)apply at all points of the object and image fields, whereby residualaberrations do not contribute any significant effect on the imagecharacteristics. In practice, a highly corrected OIS or a goodapproximation thereto, represents the target of an achievable opticalsystem design. The OIS operates under a condition of fixed numericalaperture and magnification during the measurement of an image.

Although the OIS is assumed to be corrected to the diffraction limit, asis the standard with currently available optical designs, the presentmethod may also use systems that are corrected only in an approximatesense. In these cases the minimum blur spot is aberration as opposed todiffraction limited, to within a maximum blur spot radius that is threetimes the diffraction limited value (see relation (6) hereinbelow). Inthese cases, the minimum blur spot is aberration limited as opposed todiffraction limited. Obviously, a similar but not identical set ofcriteria apply.

The present method allows a high image resolution from the OIS, asdefined per a smallest spatial feature of which it may form an image.More specifically, the image resolution is here defined as thereciprocal of a minimum resolvable feature distance in the image formedby the OIS, which may be expressed as follows:

R=1/(2δ_(r))   (3)

where δ_(r) is a minimum image feature radius (unsigned) resolvable bythe OIS.

The smallest object feature of which the OIS may form an image is apoint system object, which may be approximated, as optical theoryteaches (see for example Warren J. Smith, Modern Optical Engineering,Second Edition, 1990, McGraw-Hill, New York.), in the case of an OISthat forms diffraction-limited images at fixed numerical aperturethereof and given wavelength λ, by a small, approximately sphericalemitter located in the object plane of the OIS, with an average radiusless than δ_(R0) defined as follows:

$\begin{matrix}{\delta_{Ro} = \frac{0.61\lambda}{{NA}_{o}}} & (4)\end{matrix}$

where NA₀ is the numerical aperture of the OIS on the object side.

The image of a point object located in or away from the OIS object planeis called a blur spot, while the image of a point system object iscalled a diffraction limited blur spot or an in-focus blur spot (seeWarren J. Smith, Modern Optical Engineering, Second Edition, 1990,McGraw-Hill, New York.). Optical diffraction theory teaches that adiffraction limited blur spot corresponding to an on-axis point systemobject (located at x=y=0) has a radial intensity dependence I_(R)(r)given by an Airy disc as follows (see Warren J. Smith, Modern OpticalEngineering, Second Edition, 1990, McGraw-Hill, New York.):

$\begin{matrix}{{I_{R}(r)} = {A_{0}\left\lbrack \frac{J_{1}\left( {{kNA}_{i}r} \right)}{\left( {{kNA}_{i}r} \right)} \right\rbrack}^{2}} & (5)\end{matrix}$

where I_(R)(r) is the (Airy disc) radial image intensity distribution;A₀ is a leading constant (with dimensions of intensity); k is the wavevector (k=2π/λ); NA_(i) is the image side numerical aperture; and r isthe radial distance from the optic axis r=√{square root over (x²+y²)}.J₁ is the Bessel function of first order. The diffraction limited blurspot has a radius δ_(Ri) as follows:

$\begin{matrix}{\delta_{Ri} = \frac{0.61\lambda}{{NA}_{i}}} & (6)\end{matrix}$

For an on-axis point object displaced from the object plane of the OISby a small axial distance Δz₀, referred to as the axial defocus offsetdistance, the blur spot has a radius larger than the minimum value givenby relation (6). The condition Δz₀≠0 corresponds to a condition ofdefocus of the point object. The larger the magnitude of the axialdefocus offset distance Δ_(z0) of the on-axis point object, the largerthe effective radius of the blur spot that forms in the image plane.Optical diffraction theory teaches that the blur spot formed of anon-axis (x=y=0) point object that is displaced at the axial defocusoffset distance Δz₀ from the object plane, and has an intensity asfollows (see J. F. Power, Fresnel Diffraction Model for the Point Spreadof a Laser Light Profile Microscope, Applied Physics B 78, 693-703(2004)):

$\begin{matrix}{{h\left( {r,z} \right)}=={\frac{A_{0}}{\xi^{2}}\left\lbrack {{L_{1}^{2}\left( {\xi,\eta} \right)} + {L_{2}^{2}\left( {\xi,\eta} \right)}} \right\rbrack}} & (7)\end{matrix}$

where A₀ is a leading constant (with dimensions of intensity);

$\xi = {\frac{\pi}{2} \cdot \frac{\Delta \; z_{0}}{\delta_{0}}}$

is a defocus variable, expressed in terms of the object defocus Δz₀; andthe object depth of focus δ₀ (as defined below in relation (8)); η=kNA_(i) r is the transverse variable expressed in terms of the wavenumber k, the image side numerical aperture NAi and the radial variabler=√{square root over (x²+y²)}; and L₁(ξ, η) and L₂(ξ, η) are the Lommelfunctions, defined as:

${L_{1}\left( {\xi,\eta} \right)} = {\sum\limits_{n = 0}^{\infty}{\left( {- 1} \right)^{n}\left( \frac{\xi}{\eta} \right)^{{2n} + 1}{J_{{2n} + 1}(\eta)}}}$and${L_{2}\left( {\xi,\eta} \right)} = {\sum\limits_{n = 0}^{\infty}{\left( {- 1} \right)^{n}\left( \frac{\xi}{\eta} \right)^{{2n} + 2}{J_{{2n} + 2}(\eta)}}}$

where J_(m)(η) are Bessel functions of order m.

The radial dependence of the blur spot in relation (7) reduces to theAiry disk (relation (5) as Δz₀ approaches zero). Because the OIS is ahighly corrected optical system, the dependence of the blur spotintensity on the radial distance from the blur spot center shows novariation for off axis field points having non-zero (x, y) centralposition in the object or image field.

Optical theory (see for example Warren J. Smith, Modern OpticalEngineering, Second Edition, 1990, McGraw-Hill, New York; J. F. Power,Fresnel Diffraction Model for the Point Spread of a Laser Light ProfileMicroscope, Applied Physics B 78, 693-703 (2004)) further teaches that,for an OIS operating at fixed wavelength and aperture, there is amaximum axial defocus offset distance of a point object from the objectplane, referred to herein as the object depth of focus of the OIS δ₀,such that there is no significant difference in the blur spot radiusfrom the diffraction limited value of relation (6), defined as follows:

$\begin{matrix}{\delta_{0} = \frac{\lambda}{4{NA}_{0}^{2}}} & (8)\end{matrix}$

where λ is the wavelength of the image light, and NA₀ is the numericalaperture of the OIS on the object side.

Object point sources lying within one object depth of focus of theobject plane of the OIS produce a set of blur spots that are notsignificantly defocused and that closely approximate the diffractionlimited blur spot of relation (5). A thin object, defined as an objecthaving all points of its field confined to within a ‘z’ axial intervalof length ±δ₀ with respect to the object plane position, forms an imagethat gives a good approximation to the in-focus condition. The minimumresolvable feature distance in the image for all (x, y) field points ofa thin object is then at the diffraction limit, per relation (3) withδ_(r)=δ_(Ri).

Taking into account the fact that, in the general set up of a LPM, theobject imaged by the OIS is not necessarily thin, the present methodprovides that the object imaged in the LPM makes either a goodapproximation to, or a controlled departure from, the thin objectcondition. This ensures a high resolution in the OIS image, which, asmeasured by the blur spot radius, is either at the diffraction limit orwithin specified close limits thereof.

More specifically, the present method establishes the object in the LPM,referred to as the LPM object, in the test material by the intersectionof the source beam with the test material. This intersection defines anirradiated volume of the test material, from which light is emitted byseveral possible mechanisms, including elastic scatter, inelasticscatter, luminescence and/or blackbody emission. The LPM object is athree dimensional spatial distribution of light thus emitted in theirradiated volume, where the distribution of emitted light is linearlyproportional to the local (x, y, z) intensity of the source beam.

Standardly in the prior art, the source beam is a cylindrical GaussianTEM (0,0) laser beam, propagating along the depth direction ‘x’, forwhich the profile of source beam intensity in the test material isdefined per relation (9) as follows:

$\begin{matrix}{{I_{s}\left( {x,y,z} \right)} = {\frac{I_{0}}{{\pi\omega}^{2}}{\exp \left\lbrack {{- 2}{\left( {y^{2} + \left( {z - z_{s}} \right)^{2}} \right)/\omega^{2}}} \right\rbrack}}} & (9)\end{matrix}$

where I_(s) (x, y, z) is the spatial dependence of the Gaussian laserbeam's irradiance; ω is the mode field radius (beam spot size) along ‘x’in the xy and xz planes; I₀ is a leading constant with dimensions of anintensity; and z_(s) the center of excitation beam along the ‘z’ axisbehind the IT surface (measured from the IT surface). The Gaussian beamradius so relates to the radial distance from the beam center at whichthe Gaussian intensity profile is attenuated to a value of e⁻¹ of thevalue at the beam center, therefore defining an effective radius for thesource beam.

Also standardly in the prior art, the source beam is collimated to forma beam waist in the test material, centrally aligned at the center ofthe object field (at x=y=z=0), a laser beam waist being defined inGaussian beam theory as the axial position of the beam at which the beamradius has a minimum value ω=ω. A region of approximate collimation,referred to as the Rayleigh range, is symmetrically distributed along‘x’ with respect to the waist position. The width of the Rayleigh rangeis given from theory per the following:

$\begin{matrix}{X_{C} = \frac{{\pi\omega}_{0}^{2}}{\lambda_{s}}} & \left( {10a} \right)\end{matrix}$

where X_(C) is the confocal distance of the Gaussian beam; ω₀ is themode field radius (beam spot size) at the waist position; and λ_(s) issource beam wavelength; and:

X_(R)=2 X_(C)   (10b)

where X_(R) is the width of the Rayleigh range.

Inside the Rayleigh range, the laser beam radius deviates from the valueω₀ by no more than the maximum value ω₀√2.

As in the prior art already mentioned, in the present method, the widthof the object field along the axis ‘x’ is less than or equal to thewidth of the Rayleigh range of the source beam in the sample. The sourcebeam waist is normally aligned in the test material to coincide with thecentral optic axis of the microscope. The source beam ‘x’ axis isaligned at or within a distance along ‘z’ of ±2δ₀ of the object plane ofthe OIS.

The effective ‘z’ axial width of the LPM object is thus set by theradius of the source beam. When the radius of the source beam ismaintained smaller than the object depth of focus of the OIS, then ω₀<δ₀and the thin object condition is valid for the LPM object. All points ofthe LPM object are then in-focus. In this case the minimum resolvablefeature distance in the LPM image is a close approximate to the minimumvalue limited by diffraction, for a point system object, relation (6)and the resolution is the maximum possible limited by diffraction.

For a number of practical cases of LPM, a compression of the source beamradius, required to achieve the condition ω₀<δ₀, results in a verynarrow cylindrical irradiated volume in the test material. The LPMobject then has the form of a narrow radius cylinder oriented along thedepth axis. The field of excitation of the test material along thevertical ‘y’ dimension is then very restricted and an accordinglylimited vertical image of the object along the ‘y’ dimension isobtained. The resulting LPM image then reduces to a line image deprivedof vertical information about the object. Excitation with a line imagedoes have certain advantages as cited in the art (see U.S. Pat. No.6,614,532 for example), as when the LPM image is coupled through aspectrograph to provide a wavelength resolved line profile image.However, in the most general cases, inspection with a line source isrestrictive, and correction of this restriction is desirable.

The present method allows correcting this limitation by exciting thetest material with an elliptical source beam that approximates a thinplane sheet of excitation oriented in the (x, y) plane with a thinnestdimension thereof along the optic axis ‘z’ and a major dimension alongthe ‘y’ direction. Such a beam may be produced by an optical system suchas an anamorphic collimator. The anamorphic collimator consists of acombination of optical components, including lenses or mirrors, some ofwhich are cylindrical or toroidal optical elements (as known in the art,see for example Warren J. Smith, Modern Optical Engineering, SecondEdition, 1990, McGraw-Hill, New York, pp. 270-278), which independentlycollimate an incoming cylindrical beam along two orthogonal axes.

The axis of propagation of the elliptical source beam in the LPM isoriented along ‘x’ in the test material. The elliptic source beam has anelliptical intensity variation in the (y, z) plane, with a majorelliptic axis of the variation oriented along the vertical direction ‘y’and the minor elliptical axis oriented along the optic axis ‘z’.

The beam entering the elliptic collimator may be a Gaussian TEM (0,0)laser beam, resulting in a output beam produced by the collimator havingthe form of an elliptic Gaussian beam, with a transverse intensityprofile as follows:

$\begin{matrix}\left. {{I_{sE}\left( {x,y,z} \right)} = {\frac{I_{0}}{\pi \left( {\omega_{y},\omega_{z}} \right)} \cdot {{\exp\left\lbrack {{{- 2}\left( {y^{2}/\omega_{y}^{2}} \right)} + \left( {z - z_{s}} \right)^{2}} \right)}/\omega_{z}^{2}}}} \right\rbrack & (11)\end{matrix}$

where I_(sE) (x, y, z) is the spatial dependence of the ellipticallycollimated source beam's irradiance; ω_(y) is the mode field radius(beam spot size) in the xy plane; ω_(z) is the mode field radius (beamspot size) in the xz plane; I₀ is a leading constant with dimensions ofan intensity; z_(s) is a center of excitation beam along the ‘z’ axisbehind the IT surface (distance measured from the IT surface).

Such a beam may be controlled to have a unique waist position along thepropagation ‘x’ axis, at which the beam radii simultaneously have theminimum values ω_(y)=ω_(0y) and ω_(z)=ω_(0z) along the axes ‘y’ and ‘z’.The beam also have two orthogonally oriented Rayleigh ranges that areapplicable along axes ‘y’ and ‘z’ per relations (10a) and (10b), withω₀=ω_(0y) and ω₀=ω_(0z), to give X_(Cy), X_(Ry) and X_(Cz), X_(Rz)respectively, centered at the waists. Finally, the elliptic source beamis centered in the test material at the center of the depth ‘x’ field,at which the optic axis of the OIS intersects the ‘x’ axis.

The beam illumination provided by an elliptic laser beam according to anembodiment of the method of the present invention approximates a planesheet oriented in the (x, y) plane and centered at the object plane ofthe OIS. The extent of the ‘xy’ field of the beam imaged by the OIS islarge and rectangular or square, with dimensions that are large relativeto the beam thickness along ‘z’. Because of the small ‘z’ axialthickness, a high and controlled LPM image resolution is obtained.Furthermore, a high concentration of image light is achieved within thenarrow ‘z’ axial range, ensuring high image brightness, with a largefraction of light in the object imaged in focus.

There are practical limits on the usable ‘z’ axial thinness of theelliptic beam. These arise because the width of the region ofcollimation of the laser beam (the Rayleigh range, relations (10a) and(10b)) becomes narrower as ω_(0z), at the beam waist radius, becomessufficiently small. This places restrictions on the minimum ‘z’ axialradius of the source beam, for certain cases, such as when the OISoperates at large magnification and aperture. These restrictions arediscussed in detail hereinbelow.

The controlled ‘z’ axial thickness of the elliptic source beam allows acontrol of the minimum resolvable feature distance and thus imageresolution in the LPM image. The extent to which the ‘z’ axial thicknessof the LPM object affects the image resolution of the LPM image may bequantitatively assessed with reference to theory (see for example J. F.Power, Fresnel Diffraction Model for the Point Spread of a Laser LightProfile Microscope, Applied Physics B 78, 693-703 (2004)). Theresolution of a LPM image is defined per relation (3). The minimumresolvable feature distance δ_(r) is the radius of the blur spot formedby the LPM, denoted δ_(LPM). The LPM blur spot is the image formed bythe OIS of a single field point of the LPM object. The LPM object fieldis defined as the set of all (x, y) points of the irradiated volume fromwhich light is admitted to the image plane unobstructed by the fieldstop of the OIS.

Each field point of the LPM object field has an axial depth or distancealong ‘z’. It has been recently shown in the art (see J. F. Power,Fresnel Diffraction Model for the Point Spread of a Laser Light ProfileMicroscope, Applied Physics B 78, 693-703 (2004)), that, at fixed (x, y)field in the LPM object, the LPM blur spot forms as the integral (sum)of a set of weighted blur spots originating from all source pointsdistributed continuously along ‘z’ in the LPM object volume. At fixed(x, y), each ‘z’ axial point in the object volume contributes anindividual blur spot, which is proportionally weighted by the localvalue of the emitted source beam intensity I_(s) (x, y, z). A suitabletest material has optical properties that vary slowly in the ‘z’direction, meaning that they do not vary significantly within a distanceof ±10 δ₀ to ensure the validity of relation 11. The image that appearsin the image plane of a single field point of the LPM object is thesummation (superposition) of these weighted blur spots, as describedfrom theory as follows:

$\begin{matrix}{{h_{L}(r)} = {\frac{\Phi \; I_{0}}{{\pi\omega}_{0}^{2}}{\int_{0}^{z}{\exp\left\lbrack {{- \left( {2{\left( {z - z_{s}} \right)^{2}/\omega_{0}^{2}}} \right\rbrack}{h\left( {r,{z - z_{0}^{\prime}}} \right)}{z}} \right.}}}} & (12)\end{matrix}$

where h_(L)(r) is the point spread function for the LPM excited with aconventional cylindrical beam; φ is the efficiency of scatter orluminescence from the object volume (constant over the volume); I₀ isthe integral beam intensity (constant); ω₀ is the mode field radius(beam spot size) of the source beam; z_(s) is the distance of the sourcebeam from the IT surface; h(r, z) is a point spread function of theobjective lens (having dependence as given in relation (7) hereinabove);z₀′ is the distance of the object plane from the IT surface; and z is adistance along the optic axis.

The superposition property of axial blur spots described by relation(12) results in that a narrow source beam, with radius ω₀ close in valueto δ₀, concentrates more light close to the object plane, and producesan LPM blur spot that approximates the Airy disc or diffraction limitedblur spot of relation (5). The result is a sharp image with a bestpossible resolution limited by diffraction. A wider source beam radiusincludes more out-of-focus light in the object volume, and produces amore diffuse blur spot, with contributions originating from objectpoints significantly displaced away from the object plane, resulting ina more defocused, less resolved image.

Diffraction theory enables the calculation of an effective blur spotradius for the case of an LPM image where the source beam iscylindrical, coo is specified, and the numerical aperture, magnificationand imaging wavelength of the OIS are known (see J. F. Power, FresnelDiffraction Model for the Point Spread of a Laser Light ProfileMicroscope, Applied Physics B 78, 693-703 (2004)). The LPM blur spotradius is defined as follows:

$\begin{matrix}{\frac{\int_{0}^{\delta_{LPM}}{{h_{L}(r)}{r}}}{\int_{0}^{o}{{h_{L}(r)}{r}}} = 0.975} & (13)\end{matrix}$

where δ_(LPM) is the LPM blur spot radius and h_(L)(r) is the LPM pointspread function (see relation (12)). From relation 13, δ_(LPM) may bedetermined graphically as a radial interval that brackets 97.5% of theone-dimensional radial integral of h_(L)(r).

Table 1 summarizes the relative minimum resolvable feature distanceavailable from a LPM image in terms of the relative object thickness.The minimum resolvable feature distance for the LPM image is defined astwice the radius of the LPM blur spot δ_(LPM). The relative minimumresolvable feature distance available from the LPM image is expressed asthe ratio δ_(LPM)/δ_(Ri), which measures the LPM minimum resolvablefeature distance in multiples of the diffraction limited value fromrelation (6). The relative object thickness is measured as the ratioω₀/2^(1/2)δ₀, which expresses the source beam radius in multiples of theOIS object depth of focus. These data (see J. F. Power, FresnelDiffraction Model for the Point Spread of a Laser Light ProfileMicroscope, Applied Physics B 78, 693-703 (2004)) apply at fixed imagingwavelength over a wide range of spot radius values ω₀ in the rangecomprised between 0.5 and 500 μm, and over a full range of magnificationand numerical aperture of the OIS.

TABLE 1 ω₀/2^(1/2)δ₀ δ_(LPM)/δ_(RI) 0.54 1.12 2.72 1.40 5.44 1.96 13.683.22 19.12 3.57 27.28 3.78

Conditions that are found to ensure a high resolution in the LPM imageover a known image field along the depth axis will now be presented.

Diffraction theory and the results of Table 1 indicate that when the ‘z’axial beam dimension ω_(0z) is less than five times the object depth offocus δ₀, the minimum resolvable feature distance in the LPM image is nomore than twice the Rayleigh limited blur spot radius S_(Ri). If thiscondition is maintained in the LPM beam illumination, then a highresolution, corresponding to one half the maximum possible value for theOIS, is obtained in the LPM image. However, as ω_(0z) decreases, theRayleigh range in the (x, z) plane diminishes in length per relations10a and 10b with X_(C)=X_(Cz) and ω₀=ω_(0z). Because the beam radiusω_(0z) is controlled to a nearly constant value only within the ‘x’axial width of the Rayleigh range, the ‘x’ field width of the LPM imagefor which high image resolution is obtained, is, under illumination withan elliptical beam, set by the width of the Rayleigh range. This ‘x’field width may also be measured in units of theoretical image pixels,where the theoretical pixel width is the minimum resolvable featurewidth ΔX_(LPM), as follows:

$\begin{matrix}{{\Delta \; X_{LPM}} = \frac{X_{Cz}}{\delta_{LPM}}} & (14)\end{matrix}$

where ΔX_(LPM) is the size of ‘x’ field of the LPM (as number ofresolvable pixels); X_(Cz) is the confocal distance of the source beamin the xz plane per relation (10a) hereinabove with ω₀=ω_(0z); andδ_(LPM) is the minimum spot radius resolvable by the LPM.

Under illumination with an elliptical beam, the width of the field inthe LPM along ‘y’, for which high image resolution is obtained, is setby the beam radius ω_(0y), which may be adjusted to an arbitrarily largevalue, although a radius of 10-100 X the ω_(0z) value may be typical.The extent of the ‘y’ field may be adjusted to yield a desired lightconcentration in the LPM object, given an area to be illuminated in atest material.

Even when ω₀>5δ₀, planar illumination may still provide a benefit to aLPM. As Table 1 shows, when the object dept of focus is up to 25 timessmaller than the source beam radius, i.e. when ω₀>25 δ₀, δ_(R)approaches 4δ_(Ri), which means that the resolution is still acceptable.Optical theory teaches that in this situation, spatial resolution iscontrolled by an object focus envelope that acts to suppressout-of-focus light. Planar illumination provides a large area asdescribed hereinabove for sample inspection with enhanced concentrationof light into the illuminated volume (over the case of conventionalexcitation with a cylindrical beam), which may be of interest inpractical situations.

A LPM with elliptical illumination may be characterized as operatingunder two regimes, referred to respectively as Regime 1 and Regime 2.

Assuming that the OIS operates at a visible wavelength, Regime 1 appliesapproximately at low-to-moderate numerical aperture (object side, NA₀)and magnification (M) of the OIS, where NA₀≦0.4 and M≦20× approximately,according to the Royal Microscopical Society (RMS) standard, or othermicroscopy standard systems (DIN and JIS), which numerically approximatethe RMS standard.

Regime 2 applies approximately for numerical aperture and magnificationvalues above the limits specified by Regime 1.

In Regime 1, the size of the ‘x’ axial field is large: ω_(0z) may bemaintained within the limit ω_(0z)<2δ₀ while maintaining an ‘x’ axialfield at least 10 theoretical pixels wide, per relation (14).

In Regime 2, ω_(0z) is maintained at small values that restrict the ‘x’axial field of the LPM image to below 10 theoretical pixels in width. Inthis case, the field of the LPM image with high image resolution isapproximated by a thin strip vertically oriented along the ‘y’direction. The restricted with of the ‘x’ axial field in this case maynot alter the high resolution of two dimensional LPM image, providingmodification of the apparatus is introduced. A high resolution twodimensional LPM image of wide ‘x’ field may still be recovered underelliptic illumination, providing a series of LPM strip images isacquired with step scanning at a set of field positions along ‘x’ thatare spaced by the field width ΔX_(LPM). This produces a series ofcontiguous high-resolution LPM images that may be pieced together afteracquisition of the image series.

In addition to the above-identified conditions, the method of thepresent invention allows for a number of alternative arrangements, aswill now be described.

It is found that the spatial coherence of the source beam is notessential. While a laser may be used, a broadband radiation sourceequipped with suitable elliptical collimation optics may also be used.In the case of collimation systems used by broadband sources, the regionof beam collimation is not specifically described by a Rayleigh range asdefined in relations (10a) and (10b), but by beam collimation criteriaspecific to the individual collimator's optical design. Incoherence ofthe image radiation may not be essential, but there may be losses inimage resolution in cases where the image radiation is partially orfully coherent. Monochromaticity of the source radiation may not beessential, although it may often desirable in many applications.Monochromaticity of the image radiation may not be essential, althoughdifferent specifications of beam radius, image resolution, object depthof focus and length of source beam collimation region apply at thedifferent wavelengths imaged, which is to be taken into account.

The method according to the present invention operates over theultraviolet (200 nm) through to the near infrared (1000 nm) wavelengthrange. The invention may further have applicability to operation withX-rays and electron beams, provided suitable collimation and imagingoptics are available. The method comprises alignment of the sourcebeam's central axis in the object plane of the OIS, and may allow analignment tolerance of the beam's central axis to within ±8δ₀ of the OISobject plane, in some cases, with a possible loss of image resolution.

The method of the present invention, may further allow relaxing arequirement for a substantial parallelism between the IT surface and the‘x’ axis of the elliptically collimated beam. In this case, theelliptically collimated source beam may be directed to enter the testmaterial through the IT surface at an angle to the surface normalcomprised between zero and ninety degrees. An irradiated volume isformed from the intersection of the source beam and the test material.This irradiated volume then has a planar structure resulting fromirradiation with an elliptically collimated beam. The OIS may be alignedalong an optic axis so as to image the irradiated volume in the testmaterial along the ‘x’ axis as specified.”

Turning to the FIGS. 1-4 of the appended drawings, embodiments of anapparatus according to the present invention will now be described.

FIG. 1 schematically shows a light profile microscope apparatusaccording to an embodiment of the invention, which comprises a radiationsource 1 supplying a source beam 2.

The radiation source 1 may be a laser supplying a Gaussian TEM (0,0)beam and radiation with an output wavelength in the range from thequartz ultraviolet through to the long wavelength near infrared.Infrared radiation may be contemplated, although in this wavelengthrange the image resolution of infrared optical systems may be alimitation. X-ray sources may also be used, as discussed hereinabove.The source radiation may be monochromatic or a modified white lightsource as discussed herein below.

The source beam 2 may be approximately collimated by being propagatedthrough an anamorphic collimator, which is shown in FIG. 1 as thecombination of elements 3, 5 and 6. The anamorphic collimator forms anelliptically collimated source beam 7 with major and minor elliptic axes7 a and 7 b along axes ‘y’ and ‘z’ respectively.

In this example, the anamorphic collimator comprises two orthogonallyoriented cylindrical lenses 3 and 5, respectively oriented along the‘y’, and ‘z’ axis. The cylindrical lens 3 focuses the incoming beam 2 inthe xz plane, while the cylindrical lens 5 focuses a beam 4 emergingfrom the cylindrical lens 3 in the yz plane. The relay lens system 6forms the beam 7, which maintains collimation over an axial collimation(Rayleigh) regions in the xz plane and in the yz plane. The focallengths of the lenses 3 and 5 are selected to independently adjustradial widths along ‘z’ and along ‘y’ of the beam 7 to yield a radialwidth ω_(0y) along the major axis 7A and a radial width ω_(0z) along theminor axis 7B of the elliptical beam 7.

In the case when the source beam 2 is a Gaussian laser beam, theelliptic source beam 7 formed by the anamorphic collimator 3, 5, 6 has abeam waist 14, which is the x axial position of minimum beam radiusalong both y and z dimensions ω_(0y) and ω_(0z). The regions ofcollimation of the elliptic source beam are the Rayleigh ranges of theGaussian beam 7 in the xz and xy planes, thereby centered at the waistposition 14. The beam waist of the elliptic source beam formed by theanamorphic collimator 3, 5, 6 is normally centered inside a testmaterial 8, and the region of axial collimation of the elliptic beam isnormally of much longer dimension than the ‘x’ axial thickness of thetest material 8 to be probed.

The beam width ω_(0z), of the elliptical source beam 7 along the ‘z’direction is adjusted so that ω_(0z) is less than five times the objectdepth of focus δ₀ (relation (8)) of an optical imaging system 11. Thebeam width ω_(0y) is normally set to a value at least ten times that ofω_(0z) to provide an object field along ‘y’. The elliptically collimatedsource beam 7 intersects the test material 8 to form an irradiatedvolume 9 therein, in such a way that the region of axial collimation ofthe source beam is normally centered near the center of the testmaterial along the axis ‘x’. The elliptical beam 7 in the illuminatedvolume 9 has effective radial width dimensions ω_(0z) and ω_(0y). Theelliptically collimated source beam 7 is aligned adjacent to an imagetransfer (IT) surface 8 a oriented substantially parallel to the majoraxis 7 a of the beam 7. A radiation scattered or emitted from theirradiated volume 9 of the test material 8 is collected by the apertureof an optical imaging system (OIS) 11. The optic axis 12 of the OIS 11is aligned substantially along the direction ‘z’ and the OIS objectplane is aligned at or in proximity to the central ‘x’ axis of theirradiated volume 9 in the test material 8.

In this embodiment the OIS 11 further comprises an objective lens 11 aand a tube lens 11 b, which form an image of the irradiated volume 9 ata camera 13 aligned in the image plane of the OIS 11. The output of thecamera 13 is transmitted to a storage electronics resident computer ordigital image storage system 18, via an interface cable 17, for example.

The apparatus 10 illustrated in FIG. 1 operates in Regime 1 discussedhereinabove.

FIG. 2 illustrates another embodiment of the apparatus of the invention,which operates in Regimes 1 and 2 discussed hereinabove.

The apparatus 100 comprises a number of elements with similar componentfunctions to identically numbered elements in the apparatus 10 of FIG.1, as well as a number of additional elements.

The apparatus 100 comprises means, including an imaging lens assembly 20and alignment camera 21, for aligning the source beam in the testmaterial 8 at a known position behind the IT surface 8 a, whereby theimaging lens assembly 20 forms an image of the test material at thealignment camera 21. Furthermore, the test material 8 is provided with amicro-translation stage 23, which enables the center of the ellipticalbeam in the test material 8 to be set at a known z-axial distance behindthe IT surface 8 a. Therefore, in apparatus 100, the approximatestructure of the source beam in the test material 8 is that of a thinplane, which samples a desired (x, y) region of the sample at a knownaxial distance ‘z’ behind the IT surface 8 a. This sampling arrangementfunctions analogously to an optical microtome.

The optical imaging system (OIS) 11 further comprises amicro-translation stage 24 including a readout 24 a, which allows theposition of the object plane of the OIS in the test material 8 to beaccurately established.

A stepper motor 25, which precisely drives a translation stage 26 withsub-micrometer accuracy, enables operation of the apparatus 100 inRegime 2. A motor controller 25 b communicating with the stepper motor25 via a cable connector 25 a drives the stepper motor 25. The steppermotor 25 enables a series of contiguously spaced images to be recordedalong the ‘x’ field, where each image has a full ‘y’ field at each pointalong ‘x’. These images are read by the camera 13 and stored by thesystem 18 that uses computer software to assemble a full ‘xy’ fieldimage of the sample from the individually recorded images.

FIG. 3 illustrates a further embodiment of the apparatus of the presentinvention.

This apparatus 200 comprises elements that form a combination ofelements with similar component functions to identically numberedelements of the apparatus 10 of FIG. 1.

Apparatus 200 further comprises means for enabling the image to beformed over a narrow band of wavelengths. A liquid crystal tunablefilter (LCTF) 30, by filtering the radiation processed by the objectivelens 11 a of the OIS over a narrow band of wavelengths, typically with abandwidth of less than 1 nm, allows wavelength resolution, the objectivelens 11 a being correspondingly set to have its focus corrected atinfinity (i.e. it is a so-called infinity corrected objective). Anorder-sorting filter 31, placed ahead of the LCTF 30, selects firstorder radiation therefrom. The LCTF 30 receives operating voltages froma system of control electronics 33, through a cable interface 32. Underoperation, the LCTF 30 operating voltages are scanned to continuouslyvary the central wavelength of the LCTF element.

Apparatus 200 allows recording of wavelength resolved images.

The embodiment illustrated in FIG. 4 relates to a case of an absence ofa substantial parallelism between the IT surface and the ‘x’ axis ofpropagation of the elliptically collimated beam. This allows theinvention to be implemented for imaging of the irradiation volumewithout the requirement for obtaining or preparing a cross sectionaledge of the material.

The source laser beam 2 from the source laser 1 is directed through theanamorphic collimator 3 to give the elliptically collimated beam 7 thatis directed into the test material 8, entering through the IT surface8a. The intersection of the elliptically collimated beam 7 with the testmaterial 8 results in the irradiated volume 9. An OIS 11 is alignedalong an optic axis 12, which need not be substantially orthogonal tothe ‘x’ axis but which is shown as such in FIG. 4. The OIS is equippedwith objective 11 a and tube 11 b lenses that form an image of theirradiated volume 9 at the camera 13 aligned in the OIS image plane. Theoutput of the camera 13 is transmitted to the storage electronic system18 via the interface cable 17.

Although the present invention has been described hereinabove by way ofembodiments thereof, it may be modified, without departing from thenature and teachings of the subject invention as describes herein.

1. An apparatus for illuminating a test material in a light profile microscope, comprising: a source of radiation providing a source beam propagating along a beam axis ‘x’; an anamorphic optical means providing, from the source beam emitted by said source of radiation, a source beam elliptically collimated over an ‘x’ axial collimation region having a distance comprised in a range between micrometers and meters, and having a major elliptic axis oriented along a first transverse axis ‘y’, and a minor elliptic oriented along a second transverse axis ‘z’; a test material positioned to intersect the elliptically collimated source beam within the ‘x’ axial collimation region to form an irradiated volume, said test material comprising an image transfer (IT) surface oriented substantially parallel to the ‘x’ axis and substantially orthogonal to the ‘z’ axis, said IT surface transmitting radiation emitted from said irradiated volume in said test material; an optical imaging system (OIS) forming an image, at an image plane, of the illuminated volume in the test material from the radiation transmitted by the IT surface; said OIS defining an object plane conjugate to the image plane and aligned to contain the major elliptic axis of the collimated source beam intersecting said test material in the illuminated volume, an object depth of focus of said OIS being maintained at a value of approximately at least ⅕ of a radius of the elliptically collimated source beam along the ‘z’ axis in the axial collimation region of the elliptically collimated source beam; and image receiving means receiving the image formed by said OIS in the image plane thereof; wherein said apparatus yields a high image resolution and wide image field.
 2. The apparatus according to claim 1, wherein said anamorphic optical means includes ones of cylindrical and toroidal optical elements that independently collimate the source beam along orthogonal axes ‘y’ and ‘z’ transverse to the beam propagation axis ‘x’.
 3. The apparatus according to claim 1, wherein the IT surface transmits radiation emitted one of scattering, luminescence and blackbody emission from the irradiated volume in the test material;
 4. The apparatus according to claim 1, wherein said optical imaging system (OIS) comprises at least one of lenses, mirrors and a combination thereof.
 5. The apparatus according to claim 1, wherein said image receiving means comprises means for at least one of recording, displaying, storing and a combination thereof.
 6. The apparatus according to claim 1, wherein said source of radiation is a laser, said anamorphic optical means is an anamorphic collimator comprising a combination of at least ones of cylindrical lenses and mirrors, the irradiated volume of the test material emits luminescence, and said image receiving means is a high sensitivity CCD camera.
 7. The apparatus according to claim 6, wherein said test material emits elastic scatter from the irradiated volume.
 8. The apparatus according to claim 6, wherein said test material emits Raman scatter from the irradiated volume and said optical imaging system (OIS) further comprises an optical filter that selects a narrow range of emitted wavelengths to form the image at the CCD camera.
 9. The apparatus according to claim 6, wherein said test material emits luminescence from the irradiated volume and said optical imaging system (OIS) further comprises an optical filter that selects a narrow range of emitted wavelengths to form the image at the CCD camera.
 10. The apparatus according to claim 6, wherein said laser is a high intensity laser that thermally excites the test material's irradiated volume causing chemi-luminescence to be emitted from the irradiated volume.
 11. The apparatus according to claim 1, wherein said source of radiation is a broadband radiation source emitting polychromatic radiation, said anamorphic optical means is an anamorphic collimator comprising one of a combination of cylindrical lenses and a combination of anamorphic mirrors, the irradiated volume of said test material emits elastic scatter, and said image receiving means is a CCD camera.
 12. The apparatus according to claim 1, wherein the object depth of focus of said OIS is maintained at a value comprised between ⅕ and 5 times the radius of the elliptically collimated source beam along the ‘z’ axis in the axial collimation region of the elliptically collimated source beam.
 20. A method for illuminating a test material in a light profile microscope comprising a source of radiation, an anamorphic collimator, an optical imaging system and an image recording means, said method comprising the steps of: propagating a source beam emitted by the source of radiation along a beam propagation axis ‘x’; elliptically collimating the source beam along orthogonal axes ‘y’ and ‘z’ transverse to the beam propagation axis ‘x’ to yield a elliptically collimate source beam over an ‘x’ axial collimation region having a distance comprised in a range between microns and meters, and having a major elliptic axis oriented along the transverse axis ‘y’ and a minor elliptic oriented along the transverse axis ‘z’; intersecting the elliptically collimated source beam within the ‘x’ axial collimation region with a test material to form an irradiated volume in the test material by said intersecting, and aligning an image transfer (IT) surface of the test material substantially parallel to the ‘x’ axis of the elliptically collimated source beam and substantially orthogonal to the ‘z’ axis of the elliptically collimated source beam; collecting image radiation emitted from the irradiated volume in the test material and transmitted through the IT surface by an optical imaging system (OIS); forming an image at a fixed image plane with the OIS by aligning an object plane thereof conjugate to the image plane thereof at a central axis of the irradiated volume in the test material; maintaining an object depth of focus of the OIS at a value that is approximately at least ⅕ of a radius along the ‘z’ axis of the elliptically collimated source beam in the ‘x’ axial collimation region; and recording the image formed by the OIS in the image plane.
 21. The method according to claim 20, wherein said collecting image radiation emitted from the irradiated volume in the test material and transmitted through the IT surface by an optical imaging system (OIS) comprises collecting an image radiation emitted by one of scattering, luminescence and blackbody emission from the irradiated volume in the test material.
 22. The method according to claim 20, wherein said recording the image formed by the OIS in the image plane is done with one of a camera and an image recording means. 